The present invention relates generally to input circuits that are connected to remote input sensors or other input devices. More particularly, the present invention relates to an active circuit that limits its power consumption and consequently occupies less physical space.
Input circuits provide a physical and electrical connection between an external sensor or other input device and an industrial controller or other monitoring apparatus. Typically, input circuits are ganged together on a circuit board which is part of an I/O module, each circuit board comprising a plurality of electrically isolated input circuits. This facilitates easy and cheap construction as well as a common location for making electrical connections for proximal sensors or other input devices. Typically, the output of the input circuits are coupled to application specific integrated circuits (ASICs) which convert and transmit input data over a network link. Alternately, the input circuits may be directly coupled to an industrial controller or other monitoring apparatus which monitors the input data.
FIG. 1 illustrates an individual prior art passive input circuit 10. A sensor or other input is connected across input terminal 12 and return terminal 14. Between the input terminal 10 and the return 12 is a series combination of a current limiting resistor 16 and an LED side of an opto-coupler 22. Also included are a shunt resistor 18 and shunt capacitor 20 in parallel with the LED side of opto-coupler 22 which primarily serve to reduce the effect of spurious noise and voltage inputs significantly below the turn-on threshold of the input circuit 10. The output side of the opto-coupler 22 provides an electrically isolated input signal which is coupled to the ASIC. A diode 32 may also be included to block reverse voltage.
In many industrial automation applications the input voltage applied between terminals 12 and 14 may range between approximately 0; and 31.2 volts. Generally, a low or "off" state is represented by an input voltage between 0 and 10 volts. Conversely, a high or "on" state is represented by an input voltage between 10 and 31.2 volts. Since the opto-coupler must have enough current to turn on at the lower range of turn-on input voltage (i.e. 10 volts) the current limiting resistor cannot be too large. However, to satisfy this condition the input circuit draws a relatively large current at the upper end of the permissible input voltage range (i.e. 31.2 volts).
FIG. 3 (Line B) illustrates a relatively linear relationship between the input voltage and current of the passive input circuit. As shown, the current through the circuit 10 increases linearly when the input voltage is below the turn-on threshold of the circuit. Similarly, once the circuit is "on" the current increases linearly with the input voltage throughout the permissible input range and consequently the current limiting resistor 16 consumes more power as the input voltage increases. In particular, when the maximum input voltage of 31.2 volts is applied to terminal 12, the voltage across the current limiting resistor 16 is approximately 29.3 volts, given the opto-coupler LED turns on between 0.8 and 1.2 volts, and the current limiting resistor 16 is consuming nearly 0.5 watts of power. As a result, current limiting resistor 16 must have a relatively high power rating as compared with the other circuit components. Moreover, the size and heat dissipation of resistor 16 are increased. Also, when many input circuits are ganged together on a single circuit board the heat dissapation rating of the board often requires the usage of ceramic or iron-clad type circuit boards which is more expensive.
Although passive input circuits are relatively cheap they consume an appreciable amount of power, generate a significant amount of heat, and consume a significant amount of circuit board real estate due to the large current limiting resistor 16. Thus, there is a need for an input circuit which consumes less power and less circuit board real estate but is still relatively cheap to construct. In particular, there is a need for an active input circuit that clamps the input current within a nominal range when the input voltage exceeds the turn-on threshold to yield a circuit which consumes less power and circuit board real estate. Moreover, there is a present need for an active input circuit which consumes less power thereby enabling the usage of less expensive circuit boards when a plurality of input circuits are ganged together.